strfmon_test: Add some tests

Attempt to test the correctness of strfmon(3).

Some of them were inspired from the examples section at:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/strfmon.html

Items marked with XXX represent an invalid output.

Reviewed by: kib
PR: 267282
Github PR: #619
MFC after: 1 week